1. Related Application
The present application is related to a co-pending application of the same inventors entitled "Apparatus For Measuring Impedance Of Body Tissue" filed simultaneously herewith, Ser. No. 061,547.
2. Field of the Invention
The present invention is directed to an apparatus for making an impedance measurement of body tissue, and in particular to such an apparatus having a signal source which impresses a signal on the tissue to be measured, and means for acquiring an impedance signal from the tissue dependent on the impresses signal.
3. Description of the Prior Art
Body tissue impedance measurement devices are known wherein an impedance signal is obtained from body tissue to which a current or voltage signal has been supplied. Such known devices include an evaluation means which separates higher frequency signal components from the impedance signal so acquired. Such a unit is described, for example, in combination with frequency-controlled heart pacemakers in U.S. Pat. No. 4,303,705.
Other devices of this type are known wherein the evaluation means separates both lower frequency and higher frequency signal components from the impedance signal to identify the degree of blood loss occurring during an operation, as in U.S. Pat. No. 3,532,086. The low-frequency component is a measure of the blood volume.
Heretofore, impedance measurements in body tissue (including blood) were for the purpose of identifying mechanical volume changes of the body, for example, the volume of a beating heart or the thorax volume during respiration movement. The change in the impedance can then be employed for controlling the stimulation pulse frequency of a heart pacemaker. In simplified terms, the basis of making such an impedance measurement is the following physical relationship: EQU R=l/,(.sigma..sub.R .multidot.F)
wherein R is the impedance, .sigma..sub.R is the conductance (1/.OMEGA..multidot.cm), l is the effective electrode spacing (cm), and F is the effective line cross-section (cm.sup.2) between the electrodes.
Thus by measuring the periodic impedance fluctuation, the changes in l or F of the line path are monitored.
Thus in such known devices, a direct measurement of the metabolism of interest, which in turn is a direct measure of certain types of body stress or body changes, is not possible.